1. Field of the Invention
The present invention relates to a semiconductor device, and specifically, to a semiconductor device having an element isolation insulating film.
2. Description of the Background Art
In a semiconductor device, the surface of a semiconductor substrate is partitioned into a plurality of element forming regions by an element isolation insulating film. For example, in a semiconductor device with memory cells, the plurality of element forming regions may be separated mainly into a memory cell region, a peripheral circuitry region and the like. The memory cells are formed in the memory cell region, and the peripheral circuitry is formed in the peripheral circuitry region.
One example of such a conventional semiconductor device is described. As shown in FIG. 36, the surface of a semiconductor substrate 101 is partitioned into a memory cell region M and a peripheral circuitry region P. Prescribed P type well 107 and buried N type well 106 are formed on the surface of semiconductor substrate 101.
In memory cell region M, gate electrodes 132, 133 of a memory cell transistor are formed. In each of gate electrodes 132, 133, a floating gate electrode (polysilicon film 108) is formed on semiconductor substrate 101 with a silicon oxide film 102 interposed between them. A control gate electrode (polysilicon film 111) is formed on floating gate electrode 108 with an ONO film 109 interposed between them.
Further, drain regions 114a, 114b and a source region 115 of the memory cell transistor are formed on the surface of semiconductor substrate 101.
In peripheral circuitry region P, gate electrodes 134, 135 of a transistor for peripheral circuitry are formed. Further, source/drain regions 116, 117 of the transistor are formed on the surface of semiconductor substrate 101. A dummy gate electrode 131 is formed on an element isolation insulating film 105a. This dummy gate electrode 131 is formed simultaneously with gate electrodes 132, 133134 and 135.
An interlayer insulating film 119 is formed on semiconductor substrate 101 so as to cover gate electrodes 132, 133134 and 135, and dummy gate electrode 131. A conventional semiconductor device is structured as above.
The conventional semiconductor device above, however, involves a problem that a large number of crystal defects occur in a portion of semiconductor substrate 101 positioned close to element isolation insulating film 105a, and consequently a leakage current is increased. Accordingly, a desired operation can not be performed and the reliability of the semiconductor device is degraded.